1. Field of the Invention
The present invention relates to an optical submarine branching unit.
2. Description of the Related Art
An optical submarine cable transmission system is widely used as international and domestic communications means, which is due to its higher transmission quality such as excellence in noise and delay time, and security over satellite communications or other conventional radio communications by means of short waves. In the optical submarine cable transmission system, a plurality of optical submarine repeaters are provided at given intervals to prevent degradation in transmission characteristics of an optical submarine cable and amplify signals. Each submarine repeater is driven by a constant current supplied from a feeding device installed on the land.
Thus, the optical submarine cable transmission system is composed mainly of an optical submarine cable and a plurality of optical submarine repeaters inserted in the optical submarine cable at given intervals. The system further includes an optical submarine branching unit for branching the optical submarine cable into a plurality of transmission lines in the sea, because many optical fibers are accommodated in the optical submarine cable and wavelength division multiplexed optical signals are transmitted in each optical fiber. The optical submarine branching unit is used generally to branch a trunk cable into first and second branch cables. Such an optical submarine branching unit is installed between optical submarine repeaters or between an optical submarine repeater and a land terminal device.
A conventional optical submarine branching unit has only a function of extracting a specific wavelength. Accordingly, in the case that optical signals having different wavelengths are to be separately received by individual terminal stations, it is necessary to design an optical submarine branching unit according to wavelength. That is, many kinds of optical submarine branching unit according to the number of wavelengths to be branched must be installed. Such an increase in kind of optical submarine branching unit causes an increase in number of preliminary optical submarine branching units that must be respectively added to the existing optical submarine branching units. Further, the increase in kind of optical submarine branching unit causes another problem that different components must be manufactured for the different branching units and different tests must also be made, resulting in a reduction in manufacturability.
To solve the above problems, a branching circuit unit accommodated in the branching unit must include a plurality of optical filter circuits each having a function of reflecting light having only a specific wavelength of plural wavelengths and an optical circulator having a function of transmitting light cyclically in a given direction. Further, it is essential to develop a system in which input/output portions of all the optical filter circuits and input/output portions of the optical circulator are introduced into a pressure housing of the branching unit through feed through elements provided in cover assemblies of the pressure housing, and a required wavelength is selected outside of the pressure housing by cable jointing installation.